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<h1><a class="toc-backref" href="./more-lambda-capabilities.html#id52" name="partial-metafunction">Partial Metafunction Application</a></h1>
<p>Consider the lambda expression <tt class="literal"><span class="pre">mpl::plus&lt;_1,_1&gt;</span></tt>.  A single
argument is directed to both of <tt class="literal"><span class="pre">plus</span></tt>'s parameters, thereby
adding a number to itself.  Thus, a <em>binary</em> metafunction,
<tt class="literal"><span class="pre">plus</span></tt>, is used to build a <em>unary</em> lambda expression.  In other
words, we've created a whole new computation!  We're not done yet,
though:  By supplying a non-placeholder as one of the arguments, we
can build a unary lambda expression that adds a fixed value, say
42, to its argument:</p>
<pre class="literal-block">
mpl::plus&lt;_1, mpl::int_&lt;42&gt; &gt;
</pre>
<!-- @ apply_test = 'enum { value = mpl::apply<', ''' >::type::value };
              BOOST_STATIC_ASSERT(value == %d);'''

example.wrap(apply_test[0], 
             ', mpl::int_<3>' + apply_test[1] % 45)
compile() -->
<p>The process of binding argument values to a subset of a function's
parameters is known in the world of functional programming as
<strong>partial function application</strong>.</p>
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